AVILA BEACH – PG&E has powered down Unit 2 at its Diablo Canyon Power Plant after a migration of small jellyfish-like creatures known as sea salps.

As reported by the San Luis Obispo Tribune, southerly winds began blowing the salps into the plant’s cooling water intake cove on Tuesday. Plant operators noticed differences in water pressure at the intake structure, which meant the salps were beginning to clog the rolling screens in front of the intake.

After initially reducing power in Unit 2 to 15 percent, the problem with the animals first got better and then got worse. So, on Wednesday, the decision was made to fully power down the plant.

“I’ve been very pleased with how staff has reacted to this by putting safety first,” Ed Halpin, PG&E’s chief nuclear officer, told the newspaper.

Small jellyfish-like creatures called sea salps are in the water near Diablo Canyon.

Millions if not billions of sea salps, a one- to three-inch long transparent barrel-shaped animal that looks and feels much like a jellyfish, came ashore in the area with onshore currents. These creatures feed on plankton, and multiply rapidly.

The plant will return to full power as soon as it is safe to do so, and conditions warrant, Halpin said.

John Lindsey, a PG&E spokesman and meteorologist based in San Luis Obispo, said Friday that the winds have now changed direction in the area, and the salps should begin heading out to sea.

The Diablo Canyon intake provides seawater for cooling. It is 240-feet long, 100-feet wide and 18-feet high. It extends down 32 feet below sea level. The intake structure is backfilled by rock on three sides, and has water on the fourth (western) side.

The intake relies on four, 13,000-horsepower electric motors to pump 1.7 million gallons per minute or up to 2.5 billion gallons per day. In other words, the circulating water system provides the heat sink required for removal of waste heat in the power plant’s thermal cycle. The circulating water system is designed to provide cooling water necessary to condense the steam entering the main condenser.

A curtain wall at the front of the intake structure limits the amount of floating debris entering the intake structure. Bar racks near the front of the intake structure intercept large submerged debris. Traveling screens intercept all material larger than the screen mesh opening, which measure 3/8ths of an inch.

The intake also houses the Auxiliary Salt Water (ASW) pumps. The ocean water supply to the ASW system provides the cooling and heat absorption capability required to remove waste heat under normal and emergency conditions.

The two units of Diablo Canyon produce approximately 2,300 net megawatts of greenhouse-gas-free electricity, about 10 percent of all electricity generated in California. That’s enough to meet the needs of over three million homes in central and northern California. Unit 1 at the nuclear power plant was shut down for refueling starting on April 23.